The invention described herein relates to a method and device for generating energy from the motion of waves by conveying a fluid such as a common gas or a liquid through a series of at least two stages, the pressure of the fluid being incrementally increased as the fluid passes under the force of differential pressure between successive stages.
Due to the environmental, and financial costs of conventional fossil fuel or nuclear energy generation methods, many attempts have been made to harness waves and wind to generate energy in a useable form primarily as electricity. In the case of wave energy generation, many devices have been proposed which utilize a float which rides up and down with wave motion and a fixed or anchored member which remains relatively stationary. An air compression cylinder is introduced between the float and the anchored member, together with associated intake and output conduits and check valves in order to produce compressed air as the float rises and falls with the wave action. Variations to such conventional devices have been made to pressurize liquids or pump water to fill a water reservoir on shore. The pressurized fluid, air or water is then used to drive a conventional turbine and electric generator to convert the energy stored in the pressurized fluid into electrical energy.
U.S. Pat. No. 644,093 to Place (issued Feb. 29, 1900) describes a conventional marine air compressor as outlined above which also includes a submerged reservoir. The submerged reservoir is positioned below the float and air compression cylinder, and remains relatively stationary in deeper waters away from the active wave surface. The reservoir is anchored to the ocean floor and is connected in line between the air compression cylinder and the compressed air output conduit. A fluid filled dampening piston-cylinder arrangement is provided within the reservoir to compensate for the rise and fall of the water surface between high and low tide in ocean applications.
U.S. Pat. No. 4,754,157 to Windle (issued Jun. 28, 1988) describes several further variations upon conventional float type wave energy extraction devices including means to extract energy from both the rise and fall of the float and describing a number of such devices connected in parallel to fill a water reservoir.
Single float type units are also described in U.S. Pat. No. 1,665,140 to Master, 2,487,228 to Kriegel, 3,515,889 to Kammier, 4,203,294 to Budal et al and 4,560,884 to Whittecar whereas multiple parallel arrays of such units are described in U.S. Pat. No. 1,264,737 to Woods, 4,204,406 to Hopfe, 4,408,454 to Hagen et al, and 4,622,473 to Curry.
Conventional single unit devices and multiple arrays of units conventionally connected in parallel suffer from the disadvantage that a minimum amplitude of wave must be encountered before the pressure in the cylinder reaches a level at which useable pressurized fluid is generated.
As a result, waves having an amplitude below such a minimum do not generate any energy. The minimum amplitude is determined by the design of the air compressing cylinder, and this disadvantage is present in both single unit devices and multiple unit parallel arrays of the conventional methods and devices.
To clearly illustrate this point the following example is presented.
______________________________________ Assuming: intake pressure p.cndot. = 14 psi intended design output pressure p.sub.f = 250 psi air cylinder internal d = 10 in diameter air cylinder intake l.cndot. = 60 in chamber length initial volume V.cndot. = 1/4 .pi. d.sup.2 l.cndot. final volume V.sub.f = V.cndot.P.sup..cndot. /.sub.pf = 1/4 .pi. d.sup.2l.sub.f since PV = constant at a constant temperature solving for .sup.l.sub.f = .sup.l .cndot. P.sup..cndot. /P.sub.f = 3.36 inches ______________________________________
In summary, before compressed gas of 250 psi opens the check valve and exits the chamber, the piston must move over 56 inches or 95% of its full stroke. From the above example, it can be seen that the larger the difference .DELTA.P between intake pressure p.cndot. and output pressure P.sub.f the larger the wave and longer the air cylinder length must be before any compressed gas is outlet from the device (as represented mathematically .sup.1.sub.f =.sup.1 .cndot.P.cndot./P.sub.f). As in the above example, the outlet of the chamber has a check valve which will only open to exhaust compressed gas when the pressure of the gas is equal to or greater then 250 psi. If a wave is encountered which is less than the minimum amplitude (56.64 inches) the gas is pressurized to a level less than 250 psi and no compressed gas is outlet. In effect the potential energy stored in the compressed gas generated at less than the final design pressure is not captured since the outlet valve does not open. The potential energy of the compressed gas below 250 psi is dissipated as the wave subsides and the gas decompresses.
Lowering the design pressure will increase the volume of compressed air generated and will capture energy from waves of lesser amplitude (i.e. if P.sub.f =125 psi then .sup.1.sub.f =6.72 inches; if P.sub.f =50 psi then .sup.1.sub.f =16.8 inches). This design choice is of little practical value since with compressed gases of lower pressure higher volumes must be conducted and low pressure gases are of lesser value in driving turbines. The decrease in pressure output is not offset by the decrease in minimum wave amplitude (i.e. from P.sub.f =250 to 50 psi the minimum wave amplitude changes from 56.64 inches to 43.20 a net change of only 13.44 inches or 24%).
The conventional parallel arrays of compression units do not address this disadvantage and the total volume of energy captured is increased only by increasing the number of compression units. This solution is not cost effective given the capital cost of building an array of units and the operational cost of maintaining them at sea.
It is therefore desirable to provide a device which can capture energy from a wide range of wave amplitudes.
It is also desirable that such a device produce compressed gas or pressurized liquid in sufficient volume and at a high enough pressure to make the device economically viable.